The current wave of 3-dimensional (3D) films is gaining popularity and made possible by the ease of use of 3D digital cinema projection systems. However, the rate of rollout of digital systems is not adequate to keep up with demand, partly because of the relatively high cost involved. Although earlier 3D film systems suffered from various technical difficulties, including mis-configuration, low brightness, and discoloration of the picture, they were considerably less expensive than the digital cinema approach. In the 1980's, a wave of 3D films were shown in the US and elsewhere, making use of a lens and filter designed and patented by Chris Condon (U.S. Pat. No. 4,464,028). Other improvements to Condon were proposed, such as by Lipton in U.S. Pat. No. 5,841,321. Subject matter in both references are herein incorporated by reference in their entireties.
One lens configuration uses a dual-lens arrangement (e.g., an upper lens for projecting images for one eye, and a lower lens for projecting images for the other eye) to simultaneously project left- and right-eye images of a stereoscopic image pairs laid out above and below each other on a same strip of film. The left- and right-eye images are encoded by passing each image through respective lens assemblies with separate filters (e.g., linear polarizers, circular polarizers, interference comb filters, etc.). The filter in each lens assembly encodes a respective right- or left-eye image so that a viewer or audience wearing glasses having corresponding left- and right-eye decoding filters (can also be referred to as viewing filters) will only see projected left-eye images through the left-eye viewing filter, and projected right-eye images through the right-eye viewing filter.
Due to the optical properties of the filters, which can be different for the left- and right-eyes, respectively, the combination of the encoding filters in the projection system and the appropriately matched decoding filters in the viewing glasses introduces wavelength dependencies into the transmission spectra, such that the resulting spectra are not flat compared to a system without the encoding and decoding filters. For example, one or more of the filters may attenuate certain bands or regions in the visible spectrum differently than other bands or regions. This wavelength dependence in the spectral transmission is perceived by an audience as a discoloration (e.g., the projected image appears ‘greenish’), and is undesirable. The discoloration may be the same or different for each eye, (e.g., the left-eye takes on a yellowish cast and the right-eye takes a blue-ish cast), and can even differ spatially on the screen (e.g., points toward the center of the screen take on some cast, but points toward the corners of the screen take on even more).
These color errors detract from the 3D presentation for three reasons: First, the overall discoloration produces the impression that the 3D presentation through the glasses is of a lower quality than an ‘untinged’ projection; second, for those filter technologies that produce a color disparity between the projected images perceived by the audience's left and right eyes, there is a degree of discomfort and eyestrain for a viewer trying to accommodate the disparity; and third, a spatially varying discoloration across the screen can create a static on-the-screen artifact that renders the screen more apparent, thereby reducing or in some cases canceling the intended 3D stereoscopic effect. In the context of the discussion, discoloration can refer to the existence of the color shift or error due to the filters, and discoloration value can be used to refer to a quantity representing the amount of color shift or error introduced by the filters.
No known attempts to remedy this effect in a film-based 3D projection system. Although digital cinema projection systems may also suffer from one or more aspects of this problem, there may be an opportunity within a digital cinema server or light engine to provide a correction internally. For example, digital cinema projection system can make use of “measured color gamut data” in which primary colors that can be projected are measured (typically at the screen center) and discrepancies can be compensated for dynamically.
However, there is no such analog to this process in a film-based system. Thus, it is desirable to provide a film-based 3D presentation with improved quality (e.g., including color and brightness) that is at least competitive with that of the digital cinema presentations.
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